U.S. patent number 4,366,202 [Application Number 06/275,582] was granted by the patent office on 1982-12-28 for ceramic/organic web.
This patent grant is currently assigned to Kimberly-Clark Corporation. Invention is credited to Joseph Borovsky.
United States Patent |
4,366,202 |
Borovsky |
December 28, 1982 |
Ceramic/organic web
Abstract
Flexible ceramic/organic webs are prepared by coating, using
fixed nip techniques, a highly viscous, thixotropic material over
at least two sides of an organic, thermoplastic substrate. The
substrate, having fibers randomly arranged, highly dispersed and
bonded at the filament junction, is the type which disintegrates at
highly elevated temperatures. The resulting material is highly
loaded with ceramic particles such that greater than 70 percent of
the total weight of said web is ceramic material. Webs prepared in
accordance with the present invention can be shaped into a variety
of desired configurations and then fired at highly elevated
temperatures causing the substrate to disintegrate, to thereby form
ceramic articles having very high ceramic bonding properties.
Inventors: |
Borovsky; Joseph (Menasha,
WI) |
Assignee: |
Kimberly-Clark Corporation
(Neenah, WI)
|
Family
ID: |
23052956 |
Appl.
No.: |
06/275,582 |
Filed: |
June 19, 1981 |
Current U.S.
Class: |
442/164; 264/221;
427/289; 427/376.2; 427/412; 427/419.2; 427/419.7; 427/427;
442/417 |
Current CPC
Class: |
B28B
23/0006 (20130101); C04B 35/634 (20130101); C04B
35/63416 (20130101); C04B 35/63408 (20130101); Y10T
442/2861 (20150401); Y10T 442/699 (20150401) |
Current International
Class: |
B28B
23/00 (20060101); C04B 35/634 (20060101); C04B
35/63 (20060101); B32B 005/16 (); D04H
001/72 () |
Field of
Search: |
;428/251,244,241,268,286,281,242,283,284,311.5,312.6,312.8,317.9
;264/62,221 ;427/412,419.2,419.7,421,427,389.9 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Robinson; Ellis P.
Attorney, Agent or Firm: Fredericks; Wendell K. Herrick;
William D.
Claims
I claim:
1. An improved organic/ceramic web having high levels of ceramic
particles, capable of being shaped and then fired at elevated
temperatures in desired atmospheric environments to form high
strength ceramic articles, said web includes a thermoplastic
flexible and fibrous substrate having fibers randomly arranged,
highly dispersed and bonded at the filament junctions, said
improvement comprising:
a. a first ceramic particle coating covering a top surface of said
substrate; and
b. a second ceramic particle coating covering a bottom surface of
said substrate, said first and second coatings comprised of an
adhesive binder, ceramic fillers, and a wetting agent in an aqueous
system, said binder being the type which provides substantially
high levels of adhesion with said fillers having a high
filler-to-binder ratio admixture of about 15 to 1, said binder also
providing cohesion of said first coating to said second coating
around as well as between the randomly arranged and highly
dispersed fibers of said substrate, said adhered coatings providing
a thixotropic material having a high content of ceramic particles
covering said thermoplastic substrate providing a web that is
shapable to a desired structural configuration, during use, said
substrate being the type which will rapidly disintegrate at highly
elevated temperatures leaving said thixotropic material disposed in
said desired structural configuration, to thereby provide a shaped
ceramic article with strong ceramic bonding between said ceramic
particles.
2. A web in accordance with claim 1 wherein said fillers are
powders or fibers and selected from the group consisting of silicon
carbide, ceramic powder, iron oxide and aluminum oxide.
3. A web in accordance with claim 2 wherein said binder is a
polyvinyl alcohol resin having a molecular weight of from 90,000 to
100,000, said polyvinyl alcohol resin being employed as the binder
when it is desired to sinter the shaped web in an oxidized
atmospheric environment.
4. A web in accordance with claim 2 wherein said binder is butyl
latex, said butyl latex being employed as a binder when it is
desired to sinter the shaped web in an inert atmospheric
environment.
5. A web in accordance with claim 3 including wetting agent of a
non-ionic surfactant.
6. A web in accordance with claim 4 including a thickener suitable
for adjusting the composition to a desired viscosity.
7. A web in accordance with claim 5 or 6 wherein said viscosity of
said composition is maintained between 8,000 and 20,000
centipoise.
8. A method of forming a ceramic/organic web possessing a high
content of ceramic particles comprising the steps of:
a. providing a thermoplastic, flexible, fibrous substrate having
fibers randomly arranged, highly dispersed and bonded filament
junctions;
b. preparing an aqueous slurry of a thixotropic ceramic admixture
having a viscosity within the range of 8,000 to 20,000 cps said
slurry being comprised of a wetting agent, an adhesive binder and
ceramic fillers, said slurry having a filler-to-binder ratio of at
least 15 to 1;
c. providing at least two showers to flood a top and a bottom
surface of said substrate with said slurry;
d. providing a pair of nip rollers downstream from said showers
disposed to provide a nip of a desired gap at a spacing equal to
the thickness of a first coating and a second coating covering the
top and bottom surfaces respectively of said substrate;
e. providing a plurality of burnishing wheels downstream from said
showers disposed to provide substantially smooth surfaces of both
the first and second coatings;
f. passing said substrate past said showers to cover the top and
bottom surfaces of said substrate with said slurry;
g. passing said substrate with said slurry covering the top and
bottom surfaces through said nip rollers, said nip providing a
shear force upon said slurry so as to fill in and coat said slurry
around and between the fibers of said substrate, said slurry being
sufficiently viscous to resist running off or draining from the
fibers of said substrate;
h. drying said coated substrate to remove the aqueous content of
said coating to form said ceramic/organic web.
9. The method of claim 8 wherein said web has a thickness of from
about 5 to 20 mils.
10. The method of claim 8 wherein said aqueous slurry consists of a
binder suitable for sintering said web in an oxidizing atmospheric
environment, ceramic particle fillers and a wetting agent.
11. The method of claim 8 wherein said aqueous slurry consists of a
binder suitable for sintering said web in an inert atmospheric
environment, ceramic particles and a thickener.
12. The method of claim 10 or 11 wherein said ceramic particles
amount to at least 70% of the total web weight.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to greensheet articles and more
particularly, in a preferred embodiment, to a flexible
ceramic/organic web and methods for forming such a web that is
capable of being fused at elevated temperatures and converted to
ceramic articles of a variety of configurations.
2. Description of the Prior Art
In prior art flexible ceramic/organic web articles, it is well
known to use open-celled organic sponges or foams as carriers of
refractory ceramic particles impregnated thereon. Various
admixtures of ceramic particles and impregnating techniques have
been employed to produce ceramic/organic webs which are
particularly useful for making fused ceramic structures or sintered
parts some of which having relatively complex shapes which retain
their structural integrity over wide temperature ranges. The major
difficulty encountered by the use of these prior art methods have
been loading sufficient quantities of ceramic slurry in a structure
suitable for providing a product which is a strongly bonded
material after sintering.
One method for forming ceramic articles is described in U.S. Pat.
No. 4,075,303 issued Feb. 21, 1978 to Yarwood et al. There, a
process for producing a ceramic article comprises providing an open
celled organic polymer foam material web possessing a predetermined
permeability and resilience, and impregnating the web with a fluid
aqueous slurry of a thixotropic ceramic composition; then passing
the impregnated foam material through a pair of rollers of a
particular nip, the pair of rollers and the nip being preset to
effect temporary successive compressions, amounting to about 50 to
90% of the thickness of said material for a first pass and 70 to
90% of the thickness for the second pass. Following the combined
impregnation and compression steps, the resulting uniformly
impregnated foam material is heated to remove moisture and then to
volatilize the foam web component. The resulting article is then
ready for use preferably as a filter for molten metals or
optionally to be formed into ceramic articles by further heating to
sinter the ceramic material.
This method of forming articles, however, is primarily for forming
a permeable structure suitable for filtering molten metals. This
technique appears to be an inefficient method for providing sinter
ceramic articles because excessive amounts of work would be
required to effectively load the open-celled foam structure to the
degree of compactness required to permit forming sintered articles
that exhibit low surface flaws and high structural uniformity over
wide ranges of temperatures.
Another method for loading a foam web with ceramic material is
described in U.S. Pat. No. 3,845,181 dated Oct. 29, 1974 issued to
Ravault et al. There, the problems associated with loading foam
with a slurry of finely divided ceramic particles are addressed. It
is recognized that the strength of porous ceramic materials so
manufactured is often not very great. This is believed to stem from
the fact that during firing, the original organic foam structure
burns out to leave thin, very fragile, webs of ceramic material
webs. In the case of a reticular organic foam, ceramic tubes will
be formed; in the case of a foam with cell walls, fragile ceramic
part spheres which are very weakly bonded will occur. In either
case, the structure is not adapted to withstand loading. Hence to
solve the loading problem using foam and the slurry of finely
divided ceramic particles, certain additives are added to the foam
and/or the slurry prior to or during impregnation which are capable
of attacking the foam and disintegrating it. After impregnation
with the ceramic slurry and during drying of the materials and when
the temperature is increased, the concentration of the attacking
agent rises as a consequence of the evaporation of the slurry; the
foam is attacked rapidly, and the dried (but unfired) structure
tends to become a coherent foam-structured mix of finely divided
particulate ceramic material and organic foam breakdown
by-products, rather than a foam coated with a layer of particulate
ceramic. Hence, on firing, a solid but more coherent overall
structure is formed.
It should be recognized that foams and sponges present inherent
disadvantages as a medium for high level loading of organic filler
materials. Also, the procedural steps required to load filler
materials in open-cell structure foam are not easily adaptable to
continuous or semi-continuous mass production techniques because of
the ease by which foam and sponge materials may be compressed and
deformed.
A further prior art method for storing refractory materials on a
carrier which employs, in addition to foam, a mesh or a cloth as
the carrier of the refractory pigment is described in U.S. Pat. No.
3,111,396 dated Nov. 19, 1963 to B. B. Ball. There, a slurry is
made of a liquid such as an organic solvent or water and a finely
divided powder such as metal, metal oxide, or other metal
compounds. The slurry can include an organic binder such as a
synthetic resin. The selected organic structure is then saturated
with the slurry as thoroughly as possible. Any excess slurry is
removed from the pores or open portions of the structure by
squeezing, leaving only the matrix of the organic structure
impregnated with the slurry. Then the coated web is slowly dried at
or near room temperature e.g., for about 18 hours to assure slow
liquid removal in order to keep the powder of the slurry in place.
Such a web coating procedure appears to be a very slow process and
is not very well suited for mass production techniques for making
greensheets.
Another method for forming a particle sheet material is described
in U.S. Pat. No. 3,962,389 dated June 8, 1976 to Takase et al.
Takase et al teaches forming a ceramic greensheet material which
does not contain organic binders such as elastomers and latex,
fixatives and polymeric coagulating agents. Their method is one for
producing particle sheet material which is shapable and contains
large amounts of inorganic fine particles, wherein the method steps
comprises graft copolymerizing cationic monomer to cellulose pulp
to obtain cationic pulp causing the cationic pulp to adsorb and
coagulate the inorganic particles which has a negative interfacial
electrokinetic potential in water, and then forming the particle
sheet material therefrom by using a sheet forming machine. Although
this method of Takase et al seems to be a very highly efficient
method for loading particles in a sheet structure it does not
appear to be an effective method for mass producing greensheets
since the abrasive nature of particles would probably destroy vital
components of the sheet forming equipment after a short period of
time.
The present invention is directed to the improved techniques and
methods for forming a ceramic/inorganic web the product which
resulted from confronting and solving the basic material storing
problems as described above. In the course of the development
additional unforeseen problems were resolved as will be
apparent.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to a flexible ceramic/organic web
for use in the manufacture of ceramic articles of a variety of
configurations. In addition to being a flexible web, the present
ceramic/organic web contains high levels of thixotropic materials
which provide a composite web structure having very high ceramic
bonding strength properties that permit heat fired structural
figures to be formed therefrom possessing unusual structural
uniformity.
The ceramic/organic web includes a flexible fibrous substrate of
woven or nonwoven material and a coating of thixotropic ceramic
composition disposed over and within the open pores of the
substrate. The pores of the substrate, which are large and which
are significantly void of foreign solid material, provide the
receptacles for large quantities of the ceramic materials.
In the preferred embodiment of the web an organic, nonwoven/ceramic
web is provided by loading the organic material substrate with a
suitable coating composition in a specific manner. This coating
composition includes an admixture of a highly refractory material,
an organic binder and a spreading or wetting agent. One such high
ceramic refractory material is silicon carbide. A suitable
polymeric binder is polyvinyl alcohol. The wetting agent may be a
large number of nonionic water-soluble surfactants possessing
wetting and emulsifying properties.
This coating composition is a substantially highly viscous slurry
which enters and deposits on and between the fibers of the fiberous
substrate when suitable coating equipment is used that is capable
of providing a sufficient rate of shear force upon the material;
also the composition has that thixotropic ability to become
sufficiently viscous to resist running out or draining from around
the fibers of the substrate as the web is air dried during the
coating operation.
When the thus coated web is formed into a desired configuration for
the purpose of forming a ceramic article, the coating has suitable
shear strength to withstand reasonable folds and bends without any
deleterious effect to the web. The web in the configured form can
be fused or sintered at elevated temperatures and converted to the
desired ceramic article; the sintering causing the nonwoven web to
disintegrate. At the elevated temperature the ceramic particles are
fused forming a homogeneous ceramic structure having very high
ceramic bonding properties.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a basic saturating apparatus for impregnating flexible
substrate that includes a pair of squeeze rollers;
FIG. 2 is a block diagram representation of the coating apparatus
of the present invention;
FIG. 3 is a microscopic illustration of a portion of the carrier
substrate of this invention; and
FIG. 4 is a microscopic illustration of an edge-view of a portion
of the coated web of this invention.
DETAILED DESCRIPTION
Referring to the drawing figures, there is shown in FIG. 1 a basic
prior art saturation apparatus 2 for impregnating a flexible web 4
with a ceramic particle slurry that includes a pair of squeeze
rollers 7--7. A saturation means 6 that is used to apply the slurry
of ceramic particles onto a moving, flexible web in a manner that
causes the particles to be impregnated thoroughly into the web
structure. The squeeze rollers 7--7 are used to remove excess
slurry from the pores or open regions of the web so as to leave
only the matrix of the web structure impregnated with the slurry.
The thus coated web is carefully collected and then dried at room
temperature to prevent loss of particle content due to excessive
handling.
To provide a high volume of ceramic particles over a substrate, a
preferred embodiment of a ceramic particle loading apparatus is
depicted in FIG. 2. Illustratively, substrate 12 is a thin
open-structured sheet product. In the preferred embodiment,
substrate 12 is a spunbonded polyester sheet product comprised of
continuous-filament polyester fibers that are randomly arranged,
highly dispersed and bonded at the filament junctions.
The particle slurry may be a composition which permits forming a
greensheet to be fired in either an oxidized or in an inert
atmosphere. If an oxidized environment is to be used then the
slurry should be comprised of 1000 gms of 10% of polyvinyl alcohol
(PVA) resin; 900 gms of silicon carbide (400 or 600 mesh); and 10
gms of 25% wetting agent.
If an inert environment is to be used when firing the greensheet,
then the slurry should comprise of 483.9 gms of 62% butyl latex,
900 gms of silicon carbide and as much of a thickener as needed to
adjust the slurry to 10,000 cps.
The preferred polyester substrate is a thermoplastic material sold
under the registered trademark REEMAY by E. I. du Pont de Nemours
& Co. (Inc.) of Wilmington, Del.
The preferred PVA is a 90,000 to 100,000 molecular weight of fully
hydrolyze resin sold under the trade designation ELVANOL (71-30) by
E. I. duPont de Nemours & Co. (Inc.) of Wilmington, Del. The
silicon carbide (SIC) filler is a crystalline material sold under
the registered trade designation carborundum by Carborundum Co. of
Niagara Falls, N.Y. The wetting agent is a nonionic surfactant sold
under the trade designation IGEPAL Co. (610) by Antara Chemicals of
New York, N.Y. The butyl latex binder is one sold under the trade
designation EMD603A by Burke-Palmason Chemical Company of Pompano
Beach, Fla. The preferred thickner is one sold under the trade
designation Paragum 131 by Para-chem, Inc. of Philadelphia.
This above described particle coated web 12a can be comprised of
inorganic refractory particles of amounts totaling greater than 70%
of the total web weight.
In addition to the embodiment described above, other materials
suitable for use as substrate 12 can be open structured, organic or
inorganic, woven or nonwoven material e.g. asbestos cloth, glass
web, nonwoven polyester and nonwoven nylon. Substrate 12 may have a
thickness of from about 5 to 20 mils and a basis weight in the
range of about 25 to 200 g/m.sup.2. The fillers can along with
other forms of silicon carbide, be ceramic powder, iron oxide or
aluminum powder, the particle size may vary from 100-800 mesh.
Alternate binders may be used for both the oxidizing or inert
atmosphere environment as long as good adhesion with the filler is
maintained at high filler-to-binder ratios. Filler-to-binder ratios
may be as high as 15:1.
The viscosity of the slurry 16 should be maintained between
8,000-20,000 cps (Brookfield #4 spindle) to insure a smooth coating
application. Aqueous or solvent systems may be used, but, aqueous
systems are preferred.
Referring now to FIG. 2, there is illustrated in block diagram
form, an improved coating apparatus 10. Supply wheel 14 is used to
store and to supply portions of substrate 12 for coating. Takeup
roller 36 is used to collect a coated web 12a. A slurry transfer
system 17 is used to transfer the slurry 16 from a storage
reservoir 18 through an outlet transfer conduit 21 to a
distribution pump 20. Pump 20 is an adjustable pump which is
capable of being adjusted to permit transfer of various ranges of
viscosity of the ceramic slurry. A first slurry shower 26,
connected to pump 20 through a first transfer conduit 24, is
mounted to a frame (not shown) at a location such as to permit
flooding the top surface of substrate 12 with the highly viscous
slurry 16. Similarly, a second slurry shower 28, connected to pump
20 through a second transfer conduit 22, is mounted to a frame (not
shown) at a location such as to permit flooding the bottom surface
of substrate 12 with the slurry. A pair of nip rollers 30--30 are
rotatably mounted downstream from the showers 26 and 28 in a manner
so as to form a fixed nip 27 of a chosen gap dimension. For forming
the preferred ceramic/organic sheet the gap is set at 20 mils.
Rollers 30--30 may be spatially adjusted so as to vary the gap of
nip 27 in accordance with the desired thickness of the slurry
covering substrate 12. A collection pan 29 is disposed below the
showers 26 and 28 and the rollers 30--30 so as to collect drippings
of excess slurry from a coated web 12a. A return transfer conduit
31 is connected between pan 29 and reservoir 18 to permit returning
the excess slurry to the reservoir for future use.
A plurality of rotatable burnishing wheels 32--32 are mounted on a
frame (not shown) downstream from the nip rollers 30--30. The
burnishing wheels 32--32 are disposed so as to permit smoothing out
any blemishes which may occur in the coated web 12a. Rotating means
(not shown) are used to rotate the burnishing wheels either in the
direction or against the direction of web travel in a manner that
accomplishes effective smoothing of the particle coating. In this
preferred embodiment the nip rollers 30--30 are teflon coated to
protect the rollers from the abrasive particles in the slurry. The
burnishing wheels 32--32 are made of steel of a strength which can
sustain the abrasiveness of the slurry particles.
An air-flow heater 34 is used to dry the moving coated web 12a as
the web is passed unsupported through the heater.
The operation of coating apparatus 10 will now be discussed. The
substrate 12 is initially threaded by hand from supply wheel 14
passed the showers 26 and 28, through the nip rollers 30--30 and
the burnishing wheels 32--32 then over the mounting post 34a near
heater 34 so as to suspend the web in space and then to the take-up
reel 36.
Prior to activating the drive motor system (not shown) for take-up
reel 36, the nip 27 between the nip rollers 30--30 are adjusted
illustratively to 20 mils the desired thickness of the finished
sheet. The burnishing rollers 32--32 are correspondingly adjusted
to be coincident with the gap established with the nip rollers
32--32. Pump 20, an adjustable, through-put pump is adjusted to
permit transfer of the slurry 16 in accordance with the viscosity
of the slurry which can range from 8,000-20,000 cps.
The temperature of heater 34 is adjusted to provide drying air at a
temperature of illustratively 140.degree. C. for drying the coated
web 12a as web 12a passes through heater 34.
The slurry mixture is formed in a conventional manner and then
deposited in reservoir 18.
Immediately upon filling reservoir 18, the drive motors for take-up
reel 36 and pump 20 are activated initiating the web coating
process at a rate illustratively of 100 ft/min.; but, however, the
coating rate can be varied from 40 to 300 ft./min. depending upon
the oven efficiency to drag the heavy coating.
Showers 26 and 28 spray slurry 16 on the top surface and the bottom
surface of web 12a respectively. The slurry is transferred through
conduit 21 from reservoir 18 by pump 20 through conduits 22 and 24
to showers 26 and 28 respectively. The top coat and bottom coat, in
addition to coating the fibers of substrate 12, are linked together
through the open regions of the substrate. See FIG. 3 which shows a
50.times. magnified portion of substrate 12; the open-structure of
the substrate can readily be discerned.
The fixed nip 27 formed by nip rollers 30--30 and the burnishing
rollers 32--32 are used to smooth out the coating composition prior
to passing the coated web 12a through heater 34. In heater 34, web
12a is suspended substantially in the air chamber of heater 34 and
hot air is blown on the coated web on all sides to dry web 12a as
the web is moved through heater 34. The thus coated web has a
structural configuration as illustrated in FIG. 4 which depicts a
50.times. magnified edge view of coated web 12a.
The preferred embodiment of the coated web 12a is a ceramic/organic
web comprised of a thermoplastic skeletal substrate coated with a
thixotropic material containing high levels of refractory particles
capable of withstanding high temperatures. Such a coated web can be
formed into a variety of desired configurations for the purpose of
forming ceramic articles. The thixotropic material has sufficient
shearing strength to withstand reasonable folding and bending
without deleterious effect upon the formation of desired shapes.
The web in a configured form can thereafter be fired or sintered at
elevated temperatures illustratively 2000.degree. C. and converted
to the desired ceramic article. During the sintering the
thermoplastic skeletal substrate is disintegrated and the remaining
ceramic particles in the thixotropic coating are joined forming a
homogeneous ceramic article having very high ceramic bonding
properties and a minimal amount of void areas.
It is to be understood that the above-described embodiments are
mainly illustrative of the principles of the invention. One skilled
in the art may make changes and modifications to the embodiment
disclosed herein and may devise other embodiments without departing
from the scope and the essential characteristics thereof.
* * * * *